[Build] Use magic linker symbols to specify an @rpath-relative install name when targeting pre-stable-ABI OSes (5.1 04-24-2019 branch).#24795

Rostepher merged 2 commits intoswiftlang:swift-5.1-branch-04-24-2019swiftlang/swift:swift-5.1-branch-04-24-2019from

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